1. Field of the Invention
The present invention relates to apparatus which add modulation effects to an acoustic signal generated by an electronic musical instrument or the like, and more particularly to such apparatus which changes a degree of modulation or delay time caused by an LFO (Low Frequency Oscillator).
2. Description of the Related Art
Conventionally, apparatus which adds to an acoustic signal effects involving modulation such as a tremolo, vibrato or chorus are realized by the use of an analog circuit such as an LFO, a VCR (Voltage Controlled Amplifier) or a BBD (Bucket Brigade Device). Especially, a modulation LFO has an important roll of controlling a VCO (Voltage Controlled Oscillator) which determines notes, etc., and a VCF (Voltage Controlled Filter) which changes tones to produce effects such as vibratos. Recently, digital signal processing techniques have greatly been developed, especially, including memories and a DSP (Digital Signal Processor). For example, such a conventional modulation effect adding apparatus is disclosed in Published Unexamined Japanese Patent Application Sho 58-108583. This apparatus includes means for generating modulation information which varies with time in correspondence to a desired modulation effect, and an address counter which generates address data which varies with time at a predetermined rate. It sequentially stores digitally encoded musical signals in time series at addresses appointed by the address information, reads the musical signal stored in the past at the address indicated by the difference between the address information and the modulation information from the time series musical signals, outputs a phase or frequency modulated version of the read musical signal based on the modulation information to thereby form address data AR which fluctuates by a value indicated by the data MD' around the address "AW-AOF" spaced by the value indicated by the data AOF from the address n appointed by the address signal AW at the current time t, as shown in FIG. 21. Namely, the address data AW is modulated with data AOF and MD' and output as an address signal AR for reading the musical signal.
Since such conventional modulation effect adding apparatus is arranged to write input signals sequentially into a memory, reads them sequentially and provides an effect of delay in accordance with the difference between the write and read addresses, however, it must be arranged that the read address does not exceed the write address when the read address is modulated. For example, if the relative relationship between the conventional write address and modulated read address is modulated beyond a predetermined value, as shown in FIG. 22, the read address would exceed the write address WA, as shown by a dot-dashed line in FIG. 22. Therefore, there is the problem that the maximum value of depth of modulation must be limited by the delay time DT in non-modulation.
Since the LFO is difficult to constitute using simple logical circuits in such conventional effect adding apparatus, waveform data is frequently stored as a table in memory, so that a big capacity memory is required. Having all waveform data entities in an LFO which generates a waveform of long period is not practical because the number of data entities is immense. Thus, a method of interpolating data with limited data entities is employed. However, this method requires the use of many memories. In addition, if the LFO waveform is to be changed, the data in many memories must be rewritten, disadvantageously.
Recently, many kinds of waveforms such as sinusoidal waveforms, triangular waveforms and rectangular waveforms are often required to be generated from an LFO to add various effect sounds. In such case, the LFO which itself already requires many memories only for low frequency oscillation requires additional waveform data used for generating the respective waveforms, so that a large amount of memory is required, disadvantageously.